This renewal application proposes studies focused on the structure and function of the two beta3 integrins, alphallbbeta3 and alphaVbeta3. The target cells for analysis are platelets, endothelial cells and bone marrow derived cells. These cells are all exposed to blood and the functions of their integrins must be tightly regulated to prevent uncontrolled binding of blood-borne ligands to the integrins. Analyses will be performed to define the molecular mechanisms, which regulate the functions of the integrins and govern their contribution to the adhesive and migratory properties of these cells. The information derived from these studies will, in turn, provide insights into the complex and biologically important responses of these cells, including platelet aggregation, thrombosis, cell homing and angiogenesis. The Program consists of four projects, each directed by an independent and productive faculty member in the Department of Molecular Cardiology at the Cleveland Clinic. Dr. Edward Plow, Ph.D., will serve as Program Director and lead Project 1. This project deals with the mechanism by which specific binding partners of alphallbbeta3 and alphaVbeta3, the Kindlins and Fyn, regulate the bidirectional signaling across the integrins. In Project 2; Dr. Jun Qin will use high resolution structural approaches to determine the long sought-after structure of the heterodimeric transmembrane domains of the beta3 integrins and examine how the function of talin is regulated to induce integrin activation. In Project 3, Dr. Eugene Podrez will determine how the scavenger receptor SR-BI controls the functional activity of alphallbbeta3 in platelets, thereby providing a direct link between lipoprotein metabolism and thrombosis. Dr. Tatiana Byzova leads Project 4, which focuses on the mechanisms by which beta3 integrin regulates angiogenesis with a specific emphasis on its role in recruitment of bone marrow derived cells to sites of new blood vessel formation. Three Cores, Administrative, Cell Culture and Protein Expression, and In Vivo, provide infrastructure support. A theme which transcends all four projects is the bi-directional signaling across the beta3 integrins. The studies span from structural mechanisms to intact cells and ultimately to in vivo models to test hypotheses in a biological setting and provide comprehensive insights into responses mediated by the beta3 integrins. Cementing the Program together is the cohesive focus on a common theme, the beta{3} integrins; multiple collaborations among the Projects; and a common objective of the investigators to resolve the structural and biological mechanisms that regulate the functions of the beta B{3} integrins.